1) Field of the Invention
The present invention relates to an optical device, in particular, which is used in optical communication suitably.
2) Description of the Related Art
In an optical device using an electrooptic crystal such as, a LiNbO3 (lithium niobate) (hereinafter, simply referred to as LN) substrate, and a LiTaO2 substrate, an optical waveguide is formed in a process in which a metal film is formed on a part of the crystal substrate and thermal diffusion is applied thereon, or proton exchange is executed in a benzoic acid after applying a patterning, after this, electrodes are formed near the optical waveguide. And the optical device is used as an optical control device, which controls light signals transmitting through the optical waveguide by an electric field applied by the electrodes.
FIG. 13 is a schematic plane view showing an optical device that is used as the above-mentioned optical control device. In an optical control device 100, a Mach-Zehnder type optical waveguide 104, which is composed of an input waveguide 101, two parallel waveguides 102-1 and 102-2, and an output waveguide 103, is formed on an LN substrate 108. This optical waveguide 104 is generally adopted in the case where the optical waveguide 104 is used as an optical modulator, and coplanar electrodes, in which a signal electrode 105 is formed above the parallel waveguide 102-1 and a ground electrode 106 is formed above the parallel waveguide 102-2, are formed.
And as in this optical device 100, in the case where a Z cut substrate is used, since the change of refractive index by an electric field in the Z-direction is utilized, the signal electrode 105 and the ground electrode 106 are formed right above the parallel waveguide 102-1 and the parallel waveguide 102-2 respectively. FIG. 14 is a sectional view of the optical device shown in FIG. 13 when seen in the direction of arrows A and A′.
At this time, the signal electrode 105 is formed above the parallel waveguide 102-1 and the ground electrode 106 is formed above the parallel waveguide 102-2 by patterning. However, in order to prevent light transmitting through the parallel waveguides 102-1 and 102-2 from being absorbed by the signal electrode 105 and the ground electrode 106, as shown in FIG. 14, between the LN substrate 108 and the signal electrode 105, and between the LN substrate and the ground electrode 106, a buffer layer 107 is interposed. As the buffer layer 107, for example, SiO2 having the thickness of approximately 0.2 μm to 1 μm is used. In the case where the optical device 100 shown in FIG. 13 is driven at a high speed as an optical modulator, a traveling-wave electrode is formed by connecting a resistor to the terminals of the signal electrode 105 and the ground electrode 106, and microwave signals are inputted from the input side. At this time, the refractive index of each of the two parallel waveguides 102-1 and 102-2 is changed by +Ana and −Δnb respectively, and since the phase difference between the two parallel waveguides 102-1 and 102-2 is changed, therefore, signal light whose intensity was modulated is outputted from the output waveguide 103.
In the optical device 100 using the LN substrate 108 having the electrooptic effect shown in FIG. 13, as shown in FIG. 14, both sides of the parallel waveguides (optical waveguides) 102-1 and 102-2 in the interaction region and the region between the parallel waveguides 102-1 and 102-2 are trenched by etching and the like, and grooves 109-1 to 109-3 are formed, with this, an optical waveguide having a ridge structure is formed.
At the optical waveguide having the ridge structure, compared with an optical waveguide formed on a flat substrate without forming the grooves 109-1 to 109-3, when an electric field is applied through the signal electrode 105 and the ground electrode 106, the electric field applying efficiency to the parallel waveguides 102-1 and 102-2 can be improved, and its driving voltage can be made low. Therefore, the modulation frequency, in the case where the optical device 100 is used as an optical modulator, can be made into a broad band.
And as shown in FIG. 15, a Mach-Zehnder type optical waveguide 111, in which the parallel waveguides 102-1 and 102-2 shown in FIG. 13 are changed to curved waveguides 110-1 and 110-2, is formed on the LN substrate 108. And an optical device 114, which comprised a signal electrode 112 and a ground electrode 113 formed corresponding to these curved waveguides 110-1 and 110-2, is formed. With this structure, it is known that the size of the optical device can be made smaller than that of the optical device 100 shown in FIG. 13. Particularly, at this optical device 114, in a process in which the curved waveguides 110-1 and 110-2 being optical waveguides in the interaction region are made into optical waveguides having the ridge structure, the effect that light is shut in the curved waveguides 110-1 and 110-2 is increased, and the loss can be reduced. FIG. 16 is a sectional view of the optical device 114 shown in FIG. 15 when seen in the direction of arrows B and B′.
As existing technologies relating to the present invention, the following Patent Literatures 1 to 4 exist.
In the Patent Literature 1, a technology, in which a Mach-Zehnder type optical waveguide is formed on a flat substrate and an auxiliary ground electrode is separately formed at the outside of a ground electrode disposed facing to a signal electrode symmetrically and the ground electrode and the auxiliary ground electrode are connected electrically by a ground electrode bridge, has been described.
And in the Patent Literature 2, an optical control device, in which a signal electrode and a ground electrode are formed on two parallel waveguides of a ridge type via a buffer layer, has been described. And in the Patent Literature 3, a waveguide type optical component, in which electrodes are disposed along the side surfaces of an optical waveguide of a ridge type, has been described. And in the Patent Literature 4, a ridge waveguide, in which a part of a ridge side wall has a concave angle, has been described.
[Patent Literature 1] Japanese Patent Laid-Open(Kokai) No. HEI 3-200924
[Patent Literature 2] Japanese Patent Laid-Open(Kokai) No. HEI 10-90638
[Patent Literature 3] Japanese Patent Laid-Open(Kokai) No. HEI 4-123018
[Patent Literature 4] Japanese Patent Laid-Open(Kokai) No. 2001-4966
At the optical devices 100 and 114 shown in FIGS. 13 to 16, in the case where the ground electrode is formed by the deposition of a metal film on plural ridges including grooves between the ridges, a stress, caused by the difference of thermal expansion (the difference of the coefficient of linear expansion) between the metal film of which the ground electrode is formed and the substrate, is applied to the ridge sections (especially, the edges of the ridge sections). And the inventor of the present invention found and confirmed that the above-mentioned stress has exerted an influence on the refractive index of light transmitting through the ridge sections of the optical waveguide.
The stress applying on the ridge sections changes depending on a temperature, therefore, the influence of the stress on the refractive index of light transmitting through the optical waveguide can also be changed by the temperature. And the change of the refractive index of light depending on the temperature causes a bias voltage applying to the operating point to change depending on the change of the temperature, at the time when light is controlled by using the optical device 100 or 114. This is a problem.
FIG. 17 is a graph showing a change of the output level of light for a voltage applying to the signal electrode. Usually, the light is controlled by changing the voltage to be applied in the range of R shown in FIG. 17. However, the value of V0 being the above-mentioned bias voltage changed depending on the change of the temperature, therefore, this prevents stable optical control.
In the technologies described in the above-mentioned Patent literatures 1 to 4, a structure, in which the stress by the difference of thermal expansion (the difference of the coefficient of linear expansion) between the metal film of which the ground electrode is formed and the substrate is reduced, has not been described.